This invention relates to EMI shielding and, more particularly, to an EMI gasket structure which improves EMI shielding arrangements. The invention also encompasses a method of forming an EMI shield and a computer system employing the EMI gasket structure.
Many types of electronic circuits, including high-frequency digital circuits, produce alternating electromagnetic fields in the course of operation. Also, an electronic circuit may be sensitive to certain electromagnetic fields arising from external sources, and may fail to operate properly within the presence of such fields. To avoid problems arising from electromagnetic fields emanating from, or incident upon, an electronic circuit, such circuits are commonly protected by an electrically conductive shield or barrier. This electrically conductive shield extends around the entire circuit and is commonly referred to as an electromagnetic interference or EMI shield.
Part of the electrical energy incident on an EMI shield is reflected while part of the energy induces alternating electrical currents in the material which forms the EMI shield. These alternating currents are dissipated by eddy currents in the EMI shielding material. Thus, the EMI shield attenuates the incident electromagnetic fields so that fields emanating from circuitry within the shielded area do not interfere with external circuitry. Similarly, the EMI shielding helps prevent fields emanating from outside the shielded area from interfering with the shielded circuitry.
EMI shielding for electronic circuitry is commonly formed by a housing or enclosure associated with the particular circuitry. In a computer system, for example, the enclosure which houses the system processor, random access memory, and related devices commonly includes structures which connect together to form an EMI shield. Other shielding may be used within the primary enclosure for shielding particular circuits in the system. Regardless of whether an EMI shield is included with a component in a shielded system or stands alone, conductive gaskets, referred to as xe2x80x9cEMI gaskets,xe2x80x9d may be used to help make good electrical contact between the various components which make up the shield. EMI gaskets generally include an electrically conductive material overlaying a resilient core material which is readily compressed as the pieces which make up an EMI shield are connected together.
FIG. 1 shows a portion of a prior art EMI gasket 10 compressed between two components which make up a portion of an EMI shield. The illustrated components comprise a base portion 11 of an input/output connector 12 associated with a computer system, and a wall 14 of the computer system chassis. Chassis wall 14 includes a cut out or opening for receiving the connector 12 so that the wall and base 11 cooperate to form a portion of the EMI shield for the computer system. In order to provide better electrical continuity between chassis wall 14 and connector base 11, EMI gasket 10 is compressed between the two components as they are connected together.
However, FIG. 1 shows that a prior art EMI gasket may leave a gap 15 where a discontinuity in the components prevents the two components from perfectly abutting each other. Gaps between an EMI shielding component such as base 11 and the EMI gasket 10 may allow shorter wavelength electromagnetic energy to penetrate the shield. Such gaps are a particular problem in digital circuits which operate at high clock rates due to the short wavelength electromagnetic energy which may emanate from these circuits. At higher clock rates, clock rates over 300 MHz for example, even small gaps in the EMI shielding may be unacceptable.
It is an object of the invention to provide an EMI gasket which reduces gaps between components intended to cooperate to provide an EMI shield. It is also an object of the invention to provide a method for reducing or limiting gaps in an EMI shield. Another object of the invention is to provide a computer system with improved EMI shielding.
An EMI gasket according to the invention includes a plurality of serrations along at least one edge. The serrations are arranged side-by-side so that each may be articulated individually. That is, one of the serrations may be displaced significantly out of the plane of the gasket without affecting adjacent serrations. Thus, when the edge of the gasket is compressed between two surfaces which include a discontinuity such as a step or protuberance between them, the serration in contact with the discontinuity is displaced out of the plane of the gasket without displacing adjacent portions of the gasket. Allowing a portion of an EMI gasket to be displaced out of the plane of the gasket without affecting adjacent portions of the gasket substantially reduces or eliminates gaps which occur due to the failure of the gasket to precisely follow the contour of a component.
In the preferred form of the invention the gasket includes a resilient material which may form a core for the gasket. An electrically conductive material is associated with the resilient material and preferably comprises a cover which substantially covers the resilient core material. Serrations along an edge of the gasket are formed by making a number of spaced apart cuts along the gasket edge. Each cut extends substantially normal to the respective edge in which the cut is formed.
The method of forming an EMI shield according to the invention comprises placing the serrated EMI gasket between an electrically conductive first surface and a second surface. The method then includes compressing the gasket between the surfaces. As the gasket is compressed, at least one serration is displaced from the plane of the gasket over a protuberance on the first surface. Adjacent serrations which do not contact the protuberance remain in the plane of the gasket with no gap forming between the adjacent serrations and the first surface.
An EMI gasket according to the invention is particularly useful in a computer system using high clock rates. A computer system according to the invention includes a first structure and a second structure, at least one of which forms part of an EMI shield for the computer. A serrated EMI gasket is interposed between the first structure and second structure to form a good EMI shield despite any protuberances or other discontinuities which may be present on the structure forming part of the EMI shield. The serrations reduce or eliminate the gaps which the discontinuity would otherwise cause between the gasket material and the surface of the structure forming part of the EMI shield.
These and other objects, advantages, and features of the invention will be apparent from the following description of the preferred embodiments, considered along with the accompanying drawings.